The invention relates to a multiple stage centrifugal pump for liquids with a plurality of rotors which are arranged on a shaft and which rotate in a pump housing.
Multiple stage centrifugal pumps are used when it is desired to produce a high pressure gradient and the required forwarding amounts or the nature of the forwarded medium is such that volumetric pumps do not come under consideration. A pump manufacturer will thus first attempt to manage with few pump stages and to make the peripheral speed or the speed of rotation of such a pump large. However, high entry accelerations arise in the rotor at the suction side of the pump at the first stage, which leads to cavitation when the net positive suction head available (NPSH.sub.A) in the installation, in which the pump is set up at the momentary forwarding amount, is not greater than the net positive suction head required (NPSH.sub.R), which is required according to the pump characteristic. The value of a pump thus essentially depends on its NPSH.sub.R value being as low as possible in order that too great a demand is not placed on the plant constructor due to the magnitude of the net positive suction head.
These relationships are described in detail in the Centrifugal Pump Handbook (1st edition, July 1985 by Gebruder Sulzer AG, 8401 Winterthur, Switzerland) in Chapter 1.5 "Kavitation und Saugverhalten". The following are named as means for the improvement of the suction capacity or of the speed of rotation:
a) Use of special suction wheels in the most varied forms. PA1 b) Use of a doubly fluted rotor as 1st stage. PA1 c) Supplementary rotor. PA1 d) Auxiliary or feeder pump.
Measures b) and d) signify considerable additional costs on pump manufacture, whereas measures a) and c) signify a restriction in the characteristic field since an optimization can actually take place only for one design point of a characteristic.